As a tire wears, the volume of the tread decreases due to frictional contact with the road surface. More importantly, as the tire wears, the volume of the tire grooves decrease and the net to gross ratio increases. Eventually the tire will require replacement.
The tread elements in relief on a tire tread, such as the sipes, tread blocks and grooves, play a fundamental role in tire traction or adhesion to the road both in the transverse direction and in the circumferential direction. Traction is especially critical when travelling on wet or snowy roads. For wet conditions, the grooves act as evacuation channels trapping and evacuating water to allow tread blocks to be in contact with the ground.
It is known in the prior art to attempt to solve the degrading tread condition through the use of sunken grooves and sunken sipes. The sunken grooves, due to a limitation in manufacturing, are generally oriented perpendicular to the shoulder region. Molding elements are typically used to form the sunken grooves, and they are often difficult to remove from the tire once the tire has been cured. To make a sunken groove, the mold elements need to be rigid enough to penetrate the uncured or green rubber tread, but flexible enough to be extracted from the cured tire without damaging the tire. Another requirement is that the sunken groove needs to be large and thick enough to be efficient, which leads to a molding element so stiff that its extraction would damage the tire tread or the element itself would fail by a fatigue fracture.
Another problem with respect to the degrading tread pertains to the stiffness of the tread block. Blades are used to cut the tread block of a tire to allow the tread block softening and enhance grip. However, a blade normally has to start from the tread surface of a new tire. It is very difficult to have a sunk blade because extraction from the tire either damages the tread or the blade fails by fatigue fracture. But the blocks of a new tire are tall and sipes may be detrimental to tread stiffness and then to the handling of a new tire.
Thus it is desired to have a method and apparatus of forming sunk grooves, sunk sipes and large keyhole sipes that do not have the disadvantages described above.